1. Field of Invention
The present invention relates to a vibration isolating apparatus which is used in an engine mount or the like of a vehicle and which damps and absorbs vibrations from a vibration generating portion.
2. Description of the Related Art
In automobile engines, a vibration isolating apparatus serving as an engine mount is disposed between the engine and the vehicle body. The vibration isolating apparatus prevents vibration off the engine from being transferred to the vehicle body.
A main liquid chamber and a sub-liquid chamber are formed within the vibration isolating apparatus. Both chambers are filled with a liquid. The main liquid chamber and the sub-liquid chamber are connected to each other by a first restricting passage and a second restricting passage. The first restricting passage absorbs shake vibration (e.g., vibration of a frequency of less than 15 Hz) and the second restricting passage absorbs idle vibration (e.g., vibration of a frequency of 20 to 40 Hz) which is of a higher frequency range than shake vibration. The sectional area of the second restricting passage in a direction normal to the longitudinal direction (the longitudinal direction is the direction in which the liquid passes) is larger than the sectional area of the first restricting passage. This second restricting passage is provided with a passage opening and closing member for setting a state in which the main liquid chamber and the sub-liquid chamber communicate with each other and a state in which they do not communicate with each other.
in the above-described vibration isolating apparatus, when a vehicle travels at high speeds or the like, shake vibration is generated. Here, the second restricting passage does not communicate with the main liquid chamber due to the passage opening and closing member. Namely, the liquid moves reciprocally between the main liquid chamber and the sub-liquid chamber via the first restricting passage only. The shake vibration is absorbed by resistance when the liquid passes through the first restricting passage or by liquid-column resonance. On the other hand, when the vehicle travels at low speeds of, for example, about 5 km/h, idle vibration is generated. Here, the first restricting passage becomes clogged and the second restricting passage communicates due to the passage opening and closing member. The liquid moves reciprocally between the main liquid chamber and the sub-liquid chamber via the second restricting passage. Accordingly, the idle vibration is absorbed by resistance when the liquid passes through the second restricting passage or by liquid-column resonance.
In the conventional vibration isolating apparatus described hereinbefore, shake vibration as well as idle vibration can be absorbed. However, when the vehicle travels at 40 to 120 km/h and shaking sounds (e,g., vibration of a frequency of 60 to 200 Hz) are generated, the shaking sounds cannot be absorbed completely by the conventional apparatus.